After create RTF, try to open, got an error "Word experienced an error trying to open the file".

I have C# class that generate RTF code and savve to RTF by stores RTF code in StringBuilder and using the following code to write text as a file:

StreamWriter SW = null;
try
{
SW = File.CreateText(filepath);
SW.WriteLine(rtfCode); //string pass from StringBuilder
}
catch (Exception ex)
{
throw ex;
}
finally
{
if(SW != null)
{
SW.Flush();
SW.Close();
}
GC.Collect();
GC.WaitForPendingFinalizers();
}

The problem is I have a class to open RTF file w/ MS Word to open RTF file right away after save, sometime I'd get this error:

"Word experienced an error trying to open the file.
Try these suggestions.
* Check the file permissions for the document or drive.
* Make sure there is sufficient free memory and disk space.
* Open the file with the Text Recovery converter. "

But when I open that folder and double click same file, I can open it just fine. I did some search on google but not found much.

1. Is that mean file still in use even after I run "GC.Collect();..." command?
2. If yes, how can I check file in use or clear it from memory?
3. The RTF file need to be open right after save. Is there anyway I can work around this?

Any suggestion is welcome. Thanks.